It is well-known that a urethane prepolymer containing free isocyanate groups may be rendered nonreactive by reacting the prepolymer with a masking agent for isocyanate groups. The resulting blocked prepolymer is normally inert but may regenerate reactive free isocyanate groups upon heating. This permits the preparation of aqueous systems of urethane prepolymers.
Aqueous dispersions or emulsions of urethane prepolymers have hitherto been prepared by ball milling or emulsifying a hydrophobic blocked prepolymer in water, or by introducing a hydrophilic group such as an ionizable group or a polyoxyethylene chain into the blocked prepolymer molecule to obtain a self-emulsifying prepolymer.
Experiments have shown that these aqueous systems are not satisfactory with respect to properties such as stability against phase separation, compatibility with other water-soluble polymeric substances, water resistance and the like.
Therefore, it is a main object of the present invention to provide a thermally reactive water-soluble urethane prepolymer which is freely soluble or dispersible in water without phase separation, compatible with other water-soluble polymeric substances in an aqueous system and gives, when reacted at an elevated temperature, a polyurethane composition having improved properties.
Other objects and advantages of the present invention will become apparent from the description hereinafter.